Multiple introductions and regional spread shape the distribution of the cryptic ascidian Didemnum perlucidum in Australia: an important baseline for management under climate change.
Anthropogenic agents of ocean change such as biological invasions, overfishing, habitat destruction, pollution and ocean acidification and warming are known to have a dramatic impact on marine ecosystems worldwide. They are also intrinsically connected. In Western Australia, a "hot-plate" settlement panel system aimed at investigating the effect of ocean warming on fouling communities led to the first report of a notorious worldwide invasive ascidian species Didemnum perlucidum. This species was subsequently recorded from numerous locations along the coast and included in the Western Australia Prevention List for Introduced Marine Pests. In the present study we used microsatellite markers to determine whether these populations are the result of single or multiple introductions to Australia and if anthropogenic vectors might have facilitated its spread. By including samples collected worldwide, we further aimed to evaluate broad-scale patterns of variation and ascertain whether regional differences could be used to determine the source of introductions of D. perlucidum to Australia. Our results report an extended geographic range for D. perlucidum in Australia and worldwide, in what is the most comprehensive genetic study of this species. Our data further supports the introduced status of D. perlucidum in Australia, an introduction that our results suggest having occurred most likely as a result of multiple events with subsequent admixture. The similarity between genotypes at locations in Western Australia suggests that domestic transport plays a crucial role in shaping these populations, most likely enhanced by climatic anomalies such as heat waves. The identification of the D. perlucidum native range and sourcing of international samples from potential sites of introduction to Australia, particularly south-east Asia, remains essential to understand the presence of this species in the country. Nevertheless, during this study we were able to increase our understanding of D. perlucidum populations in Western Australia. This work provides an important baseline for D. perlucidum management and protection of high value marine areas, in Australia and worldwide.